In the laying of pipe underground such as in the laying of sewer lines, it is necessary to bore and lay the pipe casing underground at a desired percent of grade. It thus becomes necessary to monitor and control the grade of the casing as the bore is cut and the casing is simultaneously pushed through the underground bore to maintain the desired percent of grade. However, the bore and casing cannot be seen as it is formed through extended links underground and the problem of monitoring and maintaining the desired percent of grade is a problem to which considerable attention must be given in order to avoid re-boring.
Heretofore, remote hydraulic grade indicators have been utilized, such as shown in U.S. Pat. No. 3,851,716, which include a sight tube on an indicator board at a boring station connected to a water line which is affixed to the top of the casing being bored and pushed through the ground. Any deviation in the leading edge of the casing from the desired grade either up or down provides a corresponding response to the water level in the sight tube located in the boring pit. Based on the reading of the sight tube, the operator in the boring pit may pivot the head of the casing which carries a cutting head by means of a mechanical linkage. However, the problem arises that the water line connected to the sight tube must be vented on both ends and if the apparatus is utilized below the water table, water can enter the tube and interfere with the reading at the sight tube. The apparatus also has inherent vibration problems which necessitate that the apparatus be shut down to take a reading of the sight tube. The sight tube cannot be monitored simultaneously with the boring operation. With the vibrations, air locks are often create which interfere with the accuracy of the reading in the sight tube. The above inaccuracies can result in the final line being off grade often requiring re-boring. The range of this apparatus is also limited since it reaches its practical limits owing to the mechanical nature of the system. Due to the pivotable connection of the casing head utilized to steer the casing up and down, there is an open space between the casing head and the casing pipe being laid. This space creates a problem if the apparatus is utilized in soft or sandy soil whereby the soil feeds in through the space resulting in cavein.
Applicant is the inventor of U.S. Pat. No. 4,438,820 disclosing an improved percent grade sensor which includes a light emitting diode and a linear light sensor photopotentiometer arranged in a reservoir filled with a predetermined level of oil. Any changes in the percent grade of the casing head of the boring tool creates a change in the inclination of the oil level in the reservoir. In response, the light transmitted and the intensity of light received by the photopotentiometer is changed. The resulting voltage signal is converted to a digital or decimal readout of percent grade deviation which is used to steer the casing head back to a desired percent grade. While the sensor performs well, it is costly and is highly susceptible to inaccuracies caused by deviation of the casing head and sensor in a roll direction. It has been found that if four or five degrees of roll are exceeded, the reliability of the sensor is effected.
There is a considerable need for an earth boring system and percent grade sensor which is insensitive to vibration, loss of accuracy due to wear of mechanical parts, low end unit cost price, and insensitive to roll movements.
All plan grades are given in percent of grade, for example, point A to point B at 0.5 percent of grade, results in a straight grade from point A to point B with a desired slope. Percent of grade is continuous slope. The use of a water level such as disclosed in U.S. Pat. No. 3,324,564 monitors differences in elevation from the starting point to the distance of the leading edge of the bore at any time. This has to be converted into a rate of fall or slope which is practically impossible to do and maintain a highly accurate grade. The water level system is highly susceptible to vibration and requires this continuance of boring operations every few feet to allow the system to settle down for the taking of readings. While this time of slope could have deviated by as much as 4-5%. While there are many sensors which read in degrees of angular position, this is undesirable for boring applications. For example, an electronic clinometer manufactured by Schaevitz Sensing Systems, Inc. of Phoenix, Arizona under the trademark ACCUSTAR which senses angular displacement in terms of degrees. The device has been used in many robotic, aircraft, automotive, railroad, and manufacturing applications to measure angular degrees. The device includes two ground plates which sandwich a capacitor plate. Liquid and an inert gas is sealed between the capacitor plates in each half of the housing. When rotated, the sensor provides a linear variation in capacitance which may be electronically converted into degrees of angular position. However, the device does not have an output for percent of grade readings. The output of the sensor is typically a sinusoidal function which is converted into angular degrees.
Accordingly, an important object of the present invention is to provide a percent grade monitor for accurately monitoring the grade of pipe casing being bored and pushed underground which is reliable and inexpensive.
Still another important object of the present invention is to provide a grade monitor for monitoring the grade of associate mechanical equipment and machinery which is highly accurate and not susceptible to vibrations, or roll movement.
Still another important object of the present invention is to provide grade monitoring and boring apparatus which accurately senses deviations in the percent of grade of casing being bored and laid and changes the direction of the cutting head accordingly in a highly responsive manner.